A well known method of monitoring the progress of labor in a pregnant woman utilizes the hands of a medical practitioner to palpate the abdomen to assess both the strength of a uterine contraction, and the progression of the contraction down the uterus, it being known that a "normal" uterine contraction during true labor originates at the fundus of the uterus and propagates towards the cervix. Of equal importance is the patient's own evaluation as to the progress of her labor. In addition, the extent of cervical dilation is estimated via an intra-vaginal examination. However, the accuracy of these manual methods of monitoring the progress of labor are directly related to the skill of the examiner. Furthermore, these conventional techniques are non-continuous, uncomfortable for the patient, and, for any invasive procedure, pose a risk of infection.
With the advent of medical electronics, instrumentation has taken over many of these functions. Instrumentation can be automated, operated continuously, will not tire, and can, thereby, relieve the practitioner. However, the use of conventional instrumentation presents several problems.
For example, the present technology employed for measuring uterine contractions does not yield data of significant diagnostic value. The conventional contraction monitors that are known to the inventor are primarily used to detect fetal distress, by correlating the data from a uterine contraction monitor with the fetal heart rate. However, since these monitors typically obtain data at only one location on the uterus, they do not yield information as to the progression of the uterine contraction. As a result, conventional monitors yield no data as to the status of the uterine muscle itself.
There are two types of contraction monitors in general use today: external monitors and internal monitors. The drawbacks of both are described below.
Typically, the external monitor consists of a strain gauge attached with a strap to the patient's abdomen. The gauge generates qualitative data on the strength of the uterine contractions. However, absolute pressure cannot be measured by this method, since it does not directly measure the internal pressure of the uterus.
Also, external monitors are uncomfortable for the patient. If the patient does not lie perfectly still (which is often difficult for a woman in labor), the data is flawed by artifacts. Patients must also lie flat on their backs, thereby allowing the weight of the uterus to possibly compress the ascending vena cava (the vein which returns the blood to the heart from the lower body), thereby compromising both the maternal and fetal circulation, with possible deleterious results for both.
The internal monitor measures the pressure exerted by the uterus on an intra-uterine pressure transducer. This method can yield semi-quantitative data regarding the contractions in terms of pressure.
However, this method can only be used if the cervix is dilated and the membranes have ruptured. As a consequence, it can only be used in a hospital setting, with patients in active labor. This method carries with it the risk of infection, since the transducer must pass through the bacterial flora of the vagina. These organisms may then be introduced into the uterus, and there can infect both mother and fetus.
In addition, since the membranes are not intact, a closed vessel does not exist, and Pascal's Law, which states that the pressure measured at one location in a closed vessel is the same at all points in that vessel, does not apply. As a result, the intra-uterine catheter measures different pressures, depending upon where it is located. Thus, the pressure measured by the intra-uterine catheter can be considered as semi-quantitative at best. Furthermore, this method cannot be used at all in the case of placenta previa, wherein the placenta lies across the opening of the cervix.
Also, at the present time, there is no known instrumental method, which is not invasive in nature, for continuously measuring the extent of cervical dilation. That is, some type of device must be introduced intravaginally to obtain the measurement, with the attendant drawbacks referred to above.
Recognizing the above described drawbacks, medical investigators have sought a method which would yield data of greater value. Early attempts to record the electrical activity of the smooth uterine muscle have been recorded in the medical literature.
By example, L. V. Dill et al. describe, in an article entitled "The Electrical Potentials of the Human Uterus in Labor", Amer. J. Obstet. Gynecol., Vol. 52, 735 (1946), the observation that the contraction of the uterine muscle during labor is accompanied by changes in potential of low frequency and voltage.
C. M. Steer et al. describe, in an article entitled "Electrical Activity of the Human Uterus in Labor", Amer. J. Obstet. Gynecol., Vol. 59, 25 (1950), various observations of electrical activity recorded at points on the abdominal wall. Early labor is said to be associated with electrical activity at one of three usual points on the abdominal wall. As labor advances, more leads become active. These authors also note evidence suggesting a propagation of electrical activity during early normal labor.
Using internal electrodes to collect the signal, an extensive set of experiments was performed in humans by Wolfs et al. in 1979. Results of these experiments are reported in "Electromyographic Observations on the Human Uterus during Labour", Acta Obstet. Gynecol. Scand. Suppl. 90, (1979).
Because of the risks involved, and the availability of suitable animal models, such as sheep, the use of internal electrodes in human subjects has not been actively pursued. Internal electrodes are still used extensively in sheep, primarily to measure the frequency and duration of electrical activity.
In U.S. Pat. No. 4,967,761 Nathanielsz teaches the use of internal electrodes to differentiate true from false labor by measuring and analyzing the frequency of the electrical activity of the uterus.
In U.S. Pat. No. 4,256,118 Nagel teaches the use of external electrodes to measure both the electrical activity of the uterus and the fetal heart rate. Nagel describes a range of frequencies of 150 Hz to 250 Hz for the electrical activity of the uterus. It is noted, however, that this range of frequencies is typically associated with intra-uterine measurements, whereas frequencies obtained with external electrodes are primarily in the range of 0.05 Hz to 2 Hz.
In an article entitled "External Recording and Processing of Fast Electrical Activity of the Uterus in Human Parturition", Med & Bio Eng & Comput, 22, 585-91 (1984) J. Planes et al. describe the use of external electrodes to monitor the electrical activity of the uterine muscle during labor. Using an autoregression analysis, Planes et al. characterize a contraction by six parameters. Planes et al. estimate a propagation velocity of the contraction, but do not obtain any information concerning the direction of propagation, stress on the uterine musculature, or cervical dilation. Also, Planes et al. do not suggest the use of this information for diagnostic purposes.
For example, abnormal stress on any part of the uterine muscle is known to produce a marked change in the rate of progression of the electrical activity down the uterus. Abnormal stress may be due to the presence of scar tissue, such as that resulting from a previously performed cesarean section. The detection and characterization of a change in the rate of progression would thus be a diagnostic determination of great value to the practitioner.
It is thus one object of this invention to provide a method, and apparatus for accomplishing the method, whereby true labor is distinguished from false labor and other pathological conditions, by measuring the rate and direction of movement of the electrical activity of the uterine muscle.
It is a further object of this invention to provide a method, and apparatus for accomplishing the method, for diagnosing the presence of undue stress on any part of the uterine muscle by detecting abrupt changes in the rate of movement of the electrical activity of the uterine muscle.
It is a further object of this invention to provide a method, and apparatus for accomplishing the method, for diagnosing the onset of true labor by determining the direction of movement and periodicity of the electrical activity of the uterine muscle.
It is another object of this invention to provide a method, and apparatus for accomplishing the method, for utilizing a direct current offset of an electrical signal to measure the extent of cervical dilation.
A still further object of the invention provides instrumentation for remotely monitoring a pregnant mammal to aid in diagnosing: (a) true labor from false labor and other pathological conditions, (b) the presence of undue stress on any part of the uterine muscle, (c) the onset of true labor, and/or (d) the extent of cervical dilation.